You may not realize it, but arguably the most important data dump of the quarter occurred last week. On Aug. 16, institutional investors and hedge funds with over $100 million in assets under management were required to file Form 13F with the Securities and Exchange Commission.

In plainer terms, a 13F filing provides a clear snapshot of what the brightest and most-successful money managers were buying, selling, and holding in the most recent quarter (i.e., 4/1 through 6/30). Even though 13F filings are six or more weeks old, they can still add invaluable insight into what trends money managers have been buying into or shying away from.

What's particularly noteworthy about these second-quarter 13Fs is that a number of widely held stocks on online investing app Robinhood (NASDAQ:HOOD) were extremely popular buys among billionaires. Although Robinhood's retail following is known for chasing momentum plays, billionaire money managers seemed to agree that the following four stocks were worth buying hand over fist in the second quarter.

Professional brokers at their desks, with stock quotes and charts displayed on their computers.

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One of the more high-profile names that billionaire money managers couldn't stop scooping up during the second quarter is biotech stock Moderna (NASDAQ:MRNA). The 37th most-held stock on Robinhood saw aggregate ownership by 13F filers increase 9%, or 18.5 million shares.

Among billionaires, Philippe Laffont's Coatue Management, Stephen Mandel's Lone Pine Capital, and Jim Simons' Renaissance Technologies, were big buyers. They respectively added approximately 4.88 million shares, 3.99 million shares, and 1.55 million shares in Q2.

Moderna has been such a popular addition by billionaires due to the key role it's playing in fighting the coronavirus disease 2019 (COVID-19) pandemic. The company's COVID-19 vaccine, mRNA-1273, led to 94% initial vaccine efficacy in a large-scale study, and is one of only a small number of therapies to get the green light in developed markets via emergency-use authorization.

Moderna's prospects have also been lifted by variants of the disease. The rise of the delta variant brings to the light the need for booster vaccinations or perhaps annual vaccines to reduce or eliminate infection. As a trusted COVID-19 vaccine developer, we could be witnessing a shift to recurring revenue for Moderna.

However, with no other products on pharmacy shelves and a $152 billion market cap, my personal belief is that billionaires are potentially playing with fire by piling into Moderna. With an increasingly crowded field of COVID-19 vaccines, Moderna's long-term success is far from assured.

A cannabis leaf laid in the outline of the Canadian flag's maple leaf, with joints and a cannabis bud next to the flag.

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Canopy Growth

Perhaps the biggest surprise is that billionaire money managers bought Canadian marijuana stocks hand over fist in the second quarter. Canopy Growth (NASDAQ:CGC), one of the largest pot stocks by market cap, saw its total shares held by 13F filers increase 15% in the second quarter. Among billionaires, John Overdeck's and David Siegel's Two Sigma Investments, Jeff Yass's Susquehanna International, and Renaissance Technologies gobbled up about 1.46 million shares, nearly 663,000 shares, and a little over 342,000 shares, respectively.

The optimism surrounding Canopy likely has to do with the company's improved operating performance and the growing calls from certain lawmakers to legalize cannabis in the United States. As for the former, Canopy notes that it expects to attain positive adjusted earnings before interest, taxes, depreciation, and amortization (EBITDA) by the end of fiscal 2022. 

Canopy Growth is also eagerly waiting for the U.S. federal government to legalize weed. It won't enter the U.S. marijuana market until reforms are executed by Congress. With more Americans than ever wanting to see weed legalized nationally, it would appear billionaires are placing their bets on changes happening sooner than later.

Unfortunately, Canadian regulators have bottlenecked domestic cannabis supply chains by delaying licensing approval, and Canopy Growth has done itself no favors by grossly overspending on acquisitions and, at one time, overshooting on its capacity needs. It may one day be a successful pot stock, but it hasn't shown anything, thus far, to merit investors' money.

A gloved processor using scissors to trim a cannabis flower.

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Sundial Growers

Did I mention that billionaire money managers couldn't stop buying Canadian pot stocks in the second quarter? Penny stock Sundial Growers (NASDAQ:SNDL), which is the fourth most-held stock on Robinhood, was especially popular. In aggregate, 13F filers more than doubled their stakes to 152.6 million shares, with Renaissance, Susquehanna, and Two Sigma devouring shares. Respectively, this trio added 64.37 million shares, 10.84 million shares, and 6.85 million shares in Q2.

Many of the catalysts that are expected to light a fire under Canopy Growth also apply to Sundial. This means billionaires are potentially betting on U.S. legalization, and for Sundial's operating performance to improve.

However, billionaire money managers are probably attracted to Sundial's massive cash pile, too. Sundial had approximately 1.2 billion Canadian dollars (about $933 million U.S.) in cash, marketable securities, and long-term investments with no debt, as of Aug. 9, 2021.  With such an insane cash buffer, successful money managers probably feel safe putting their money to work in Sundial.

But among the stocks on this list, Sundial may well be the least palatable investment. Management has been burying its investors with share-based offerings to raise capital since last October. In the nine months between Sept. 30, 2020 and June 30, 2021, Sundial's outstanding share count has ballooned from 509 million to over 2 billion. With management showing little regard for its shareholders, I'd suggest avoiding Sundial like the plague.

Two women holding hands and carrying luggage while checking into a bed and breakfast.

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The fourth and final Robinhood stock that billionaires bought hand over fist in the second quarter is hosting and travel platform Airbnb (NASDAQ:ABNB). The number of shares held by 13F filers rose 98% to over 158 million, with three billionaire money managers adding significant stakes. Susquehanna, Larry Fink's BlackRock, and Ken Griffin's Citadel Advisors respectively bought around 3.66 million shares, almost 1.61 million shares, and just over 1.25 million shares in Q2.

What makes Airbnb so intriguing is the way it can dually disrupt the hotel and travel industry. The company's platform has more than 4 million global hosts -- yet this represents just a fraction of households worldwide. As homeowners become aware of the cash flow potential tied to hosting, the number of bookings for Airbnb could soar.

Likewise, Airbnb isn't simply complacent focusing on hosting. The company's Experiences segment works with local experts to lead travelers on adventures. By potentially partnering with experts and businesses in destination hot spots, Airbnb has the opportunity infiltrate all aspects of the travel industry.

Maybe the biggest selling point of all is that Airbnb's long-term stays (defined as stays of at least 28 nights) have been its fastest-growing category by trip length.  This demonstrates that the Airbnb platform will have plenty of staying power, long after the pandemic has ended. Of the four Robinhood stocks listed here, it's the unquestioned best buy.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.